Method and apparatus for detecting the respiratory activity of a person

ABSTRACT

A method and a device are provided for detecting the respiratory activity of a person and for controlling the time progression of breathing gas pressure, especially in accordance with physical parameters and considering parameters indicating the momentary physiological condition of the breathing person. The device for detecting the respiratory activity of a person has at least one sensor that provides a first signal indicating the breathing gas flow, wherein at least one signal processing device is provided for processing the first signal. The signal processing device is configured in such a way that said device determines a reference-relation on the basis of the first signal detected during a first time interval. On the basis thereof, the device determines a correlation-relation between the reference-relation and the first signal. The device generates an output signal indicating the respiratory activity and/or the physiological condition of the breathing person by considering at least the correlation-relation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/454,835, filed Jun. 19, 2006, now U.S. Pat. No. 7,934,500, which is acontinuation of U.S. application Ser. No. 10/070,346, filed Jun. 21,2002, now U.S. Pat. No. 7,089,936, which claims the benefit of PCTApplication Serial No. PCT/EP01/07574, filed Jul. 2, 2001, which in turnclaims the benefit of German Application Serial No. DE 100 31 079.6,filed Jun. 30, 2000, each incorporated herein by reference in itsentirety.

FIELD OF TECHNOLOGY

The present invention is directed to a device for the detection of therespiratory activity of a person as well as for controlling the timerelated course of respiratory gas pressure particularly in accordancewith physical parameters and parameters indicative with respect to theactual physical condition of a respirating person. The present inventionmay be applied particularly in the field of sleep medicine fordiagnosing and/or treating sleep related breathing disorders by positivepressure respiration (CPAP-Therapy). Further, the present inventionaddresses a method for controlling a respiratory gas pressure inconnection with excess-pressure respiratory gas supply.

BACKGROUND OF TECHNOLOGY

CPAP-Therapy (Continuously Positive Airways Pressure-Therapy) affordsprevention of sleep related breathing disorders in a physiologicallywell accepted manner.

By means of respiratory gas supplied at a defined elevated pressurelevel above ambient pressure a pneumatic splinting of the upper airwaysmay be achieved to effectively prevent potential obstructions in thisregion—or to afford sufficient Oxygen supply towards the patient in caseof temporarily contraction of said upper airways. To achieve highphysiological acceptability it is usually envisaged to adjust a lowrespiratory pressure level affording sufficient pneumatic splinting ofthe upper airways. However, it has become evident that aforesaid lowrespiratory pressure level is subject to significant variations.Experiments have been made by using so called AUTO-CPAP devices whichfor example automatically increase the therapy pressure upon occurrenceof snoring sounds, to take these variations in required CPAP-pressureinto account. Further CPAP-devices are known for detecting the timerelated course of the breathing gas flow and analyzing same with respectto features indicative with respect to airway obstructions. In case ofsuch airway obstructions an increase of the therapy-pressure istemporarily administered.

Also there are known Auto-CPAP devices determining the presentphysiological condition of a patient by means of pressure pulses appliedto the respiratory gas supplied via a breathing gas conduit wherein forexample on the basis of an impedance detection the present degree ofobstruction may be concluded.

From EP 0 612 257 B1 there is known a system for generation ofcontinuously positive respiratory gas pressure, which system changes thepressure level of the gas supplied to the patient in a defined manner,and which analyses changes of the airflow profile that may go alongtherewith.

With respect to the pressure control concepts applied so far forautomatic patient-related adjustment of the breathing gas pressure thereexists a problem in that the changes of the respiratory pressureadministered thereby are not universally accepted by the respectivepatients. Further there exists a problem in that the known auto-CPAPsystems start to react on significant breathing disorders only.

SUMMARY OF TECHNOLOGY

It is an object of the present invention to provide a device for thedetection of the respiratory activity as well as for the provision ofphysical parameters during administration of a respiratory gas to apatient that allows a precise determination of the physiological stateof the patient.

According to the present invention this object is performed by a devicefor detecting breathing activity of a person comprising at least onemeans for supplying a first signal indicative with respect to breathinggas flow; and at least one signal processing means for processing saidfirst signal, wherein said signal processing means being construed so asto generate a reference relation on the basis of said first signaldetected over a first time period, and a correlation-relation betweensaid reference-relation and said first signal, said signal processingmeans being further construed so as to generate on the basis of anobservation of at least said correlation-relation an output signal whichis indicative with respect to the breathing activity, in particularclassifying same.

This affords in an advantageous manner an extremely exact classificationof the respiratory activity of the respirating person and, basedthereon, meeting the patients physiological state, a precise setting ofthe respiratory pressure in a convenient manner without disturbing thenatural sleep behaviour. The pressure control based on the preciseclassification or detection of the respiratory activity provides aclearly improved acceptance of therapy and allows a far sightedadjustment of the breathing gas pressure, which may prevent occurrenceof potentially occurring airway obstructions with a high likelihood.

On the basis of the determination-concept according to the presentinvention it might be possible in an advantageous manner to ensure thata patient-specific setting of the breathing gas pressure adjusted by arespective CPAP-device is achieved with high reliability and withoutparticular diagnostic efforts. On the basis of the determination conceptaccording to the present invention it is further enabled to dispensefrom active variation of the breathing gas pressure as it was so farnecessary for the supervision of the physiological state, and todetermine the physiological state of the patient without arbitrarilyadjusted pressure experiments.

According to a preferred embodiment of the present invention, the lengthof a first time period for determining the reference relation isdetermined so as to extend over at least two respiration cycles. It ispossible to define the generation of the reference relation via acriteria-array. This criteria-array preferably includes a plurality ofentries by which it is determined how the reference relation isgenerated from the first and second detected signals. It is possible forexample to determine certain features of the reference relation byprocessing said first and second signals over a period which exceeds ashorter observation period for setting other features of said referencerelation.

According to a particularly preferred embodiment of the presentinvention there is provided at least one filter-means for filtering thefirst and/or second signal with respect to a predeterminedfrequency-range. This affords to extensive suppression of certaindetection-related noise impacts.

According to a further preferred embodiment of the present invention thesignal processing means includes at least one smoothing means forsmoothing said reference relation by application of predeterminedsmoothing criteria. According to a preferred embodiment, said smoothingcriteria are set adaptively. It is also possible to select presetsmoothing criteria for certain respiratory states, or to adapt thesmoothing criteria to the detected respiratory state.

Preferably the parameters of the filter means are adaptively adjusted.The adaptation behaviour may preferably determined by input ofrespective parameters.

According to a particularly preferred embodiment of the presentinvention at least one of the aforementioned smoothing means isconstrued in such a manner that same effects smoothing on the basis ofstatistic methods.

The generation of output signals which are indicative with respect tothe respiratory activity by means of said signal processing means iscarried out in accordance with a preferred embodiment of the inventionon the basis of a threshold observation. For this a thresholdobservation means processing threshold criteria in particularzero-crossings is preferably integrated into the signal processingmeans. Preferably, the signal processing means further includes countermeans for counting accomplishment of predetermined criteria within a settime period. The time periods are preferably variably adapted to thepresent respiratory state.

The detection of signals indicative with respect to the breathing gaspressure may be carried out for example by means of a pressure sensorwhich is integrated into a respective CPAP-device and which detects forexample via a sensing tube the static pressure within a region of abreathing mask applied to a patient. The signals indicative with respectto the breathing gas flow may be determined for example via a sensingshield arrangement provided in a breathing gas supply path.

By means of the device proposed according to the invention or on thebasis of the analysis procedure carried out by said device a robustdetection of each respiratory cycle of the respirating person isaccomplished. In an advantageous manner the transition from theinspiratory phase into the expiratory phase happens via a characteristicflank on the basis of which a secure detection of each breathing cycleis enabled. In a preferred manner the first derivation in time isestimated. The local extremes of the estimated first derivation of theflow-function correspond to the maximum inclination of the respiratoryflow during transition between inspiration and expiration. Beginning inthe expiration phase the starting point of Inspiration is detected inthat a search through the preceding extreme of the estimated secondderivation is carried out. Further preferred embodiments of theinvention are subject of the dependent claims.

The length of the first time period is preferably set so as to extendover at least two breathing cycles. Preferably a second means isprovided for provision of a second signal indicative with respect to thedynamic and/or static pressure of the respiratory gas. In a preferredmanner there is provided at least one filter means for filtering ordamping the first and/or second signals.

The signal processing means preferably includes a smoothing means, forsmoothing the reference relation by use of selected smoothing criteria.Said smoothing criteria are preferably adaptively changed. The signalprocessing means preferably includes a smoothing means for smoothing ordamping said reference relation.

At least one of said smoothing means is preferably construed so as toeffect smoothing on the basis of statistical solution-statements. Thesignal processing means preferably includes a threshold considerationmeans for evaluating said correlation-relation with respect to thresholdcriteria in particular zero crossing. The signal processing meanspreferably includes a counting means for counting performance ofpredetermined criteria within a preset period of time. The filter-and/or smoothing parameters are preferably adaptively fitted.

The object of the present invention as mentioned at the beginning isfurther solved by a device for supplying respiratory gas to a patient atexcess-pressure via a feeding means for feeding said respiratory gas anda detection means for detecting at least the breathing gas pressureand/or the breathing gas flow, characterized by a signal processingmeans generating a reference relation on the basis of the detectedsignals and which is setting the breathing gas pressure on the basis ofa correlation between said reference relation and the present breathingpatterns.

The object mentioned at the beginning is further also solved by a methodfor controlling the respiratory gas pressure during CPAP-therapy, bydetecting signals indicative with respect to the breathing gas pressureand the breathing gas flow, wherein on the basis of the time relateddynamic of the measuring values of pressure and respiratory gas flow thepresence and/or degree of a flow limitation is detected and thebreathing gas pressure is controlled accordingly.

In one embodiment, the time-points of the beginning of Inspiration-and/or Expiration are determined in consideration of the inclination ofa curvature portion of the gas flow by using statistic smoothing methodsand wherein a significant variation of the distance between the ends ofInspiration- or Expiration is determined with respect to a number ofsubsequent breathing cycles.

In a advantageous manner irregularities within the breathing gas floware detected by comparing the present breath with timely precedingbreathings by application of statistical dependency measurementsPreferably correlation-coefficients and/or mutual-informations aredetected as measurements of dependency.

Preferably a correlation relation between a reference function and apresent breathing flow is generated, wherein in case of to littlestatistical dependency between the present breath and the timelypreceding breath the respiratory pressure is adjusted accordingly.

Preferably groups of breathings are standardized via affinetransformation wherein the average curvature of the standardized breathis used for detection of probably existing flow limitations.

Further the object mentioned at the beginning is also solved by a methodfor controlling the breathing gas supply pressure during CPAP-therapy bydetection of the sleeping position of the patient, in particular thehead-position, and/or torsi-position or neck-torsion-degree and whereinthe respiratory target pressure and/or the pressure controlcharacteristic of the breathing gas supply is set in dependency of thosedetections.

According to a further aspect of the present invention the objectmentioned at the beginning is solved by a method for controlling thebreathing gas supply during CPAP-therapy including detection of a signalindicative with respect to breathing gas flow, and subjecting thissignal a correlation-analysis on the basis of an adaptively actualizedreference function, wherein on the basis of the results of thecorrelation analysis the physiological state of the patient is typified,wherein with respect to the control of the respiratory gas pressure, inparticular with respect to setting a respiratory target pressure, thecontrol characteristic of a respiratory gas pressure control means isadapted.

Preferably there are provided several pressure control modes adapted forselected sleep-stages of the patient. The sleeping position of thepatient, in particular the head- and/or torsi-position, and/or the necktorsion degree are preferably detected in association herewith and thebreathing gas target pressure and/or the pressure control characteristicof supplying respiratory gas is set in consideration of these detectionsalso.

Further the object mentioned at the beginning is also performed by amethod for controlling the supply of respiratory gas pressure duringCPAP-therapy, including detection of a first signal indicative withrespect to breathing gas flow, wherein this signal subjected to acorrelation analysis based on an adaptively actualized referencefunction, wherein on the basis of the results of said correlationanalysis a physiological state of the patient is typified, wherein independency of the result of typification the breathing gas pressurecontrol is adjusted in such a manner, that same adjusts substantiallyequal static respiratory gas pressure values for inspiration andexpiration within a mask region, —or different mask pressure values forinspiration and expiration (bilevel-mode).

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and features will be apparent from the followingdescription with reference to the drawings in which:

FIG. 1 shows (top) a data-portion of a flow graph of a patient duringNREM2; (middle) a high vertical line indicating the end of inspiration,a low line indicating the start of inspiration; (bottom) the firstderivation in time of the flow-graph on the basis of which the end andthe beginning of inspiration may be detected

FIG. 2 shows (top) a data portion of a flow graph of a patient duringNREM 2; (middle) the last breathing cycle of the data sequence aboveselected as reference relation for the breathing pattern; (bottom)correlation between the data portion above (reference relation) and theflow pattern in the midst;

FIG. 3 shows (top) a data portion of a flow graph of a patient duringNREM2; (bottom) the average difference of the maxima of correlation fromthe value 1;

FIG. 4 shows (top) a data portion of the flow graph of a patient duringREM; (bottom) average difference of the maxima of correlation from thevalue 1;

FIG. 5 shows (top) a data portion of the flow graph of a patient;(middle) associated CPAP-pressure graph; (bottom) variance of the CPAPsignal per breathing cycle.

FIG. 6 is a schematic view of a device according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF ILLUSTRATED EXAMPLES

In FIG. 1 the top graph displays 45 seconds of a flow graph of a patientat NREM2-sleep stage. The lower graph of this figure shows an estimatedfirst derivation of the flow graph. Between both graphs the herebyautomatically detected transition points are indicated by verticallines.

FIG. 6 is a schematic view of a device 10 constructed according to anembodiment of the present invention. Device 10 can be used to carry outthe processing described below in relation to FIGS. 1-5. Device 10includes a flow generator 15, a patient interface 20, e.g., a breathingmask, and a breathing gas conduit 25 to deliver pressurized gas from theflow generator 15 to the patient interface 20. Flow generator 15typically includes a detector 30 to produce a signal relating tobreathing gas pressure (e.g., via a pressure sensor) and/or breathinggas flow (e.g., via a flow sensor or meter). Flow generator 15 includesa processor 40, e.g., in the form of a CPU, to receive input signalsfrom the detector 30. Processor 40 is adapted to generate areference-relation on the basis of the detector signal to adjust thebreathing gas pressure on the basis of a correlation-relation betweenthe reference-relation and a prevailing breathing pattern of thepatient.

For differentiation between stable and non-stable respiration, ameasurement of similarity of a plurality of successive breathing cyclesis considered. The height of a cross-correlation-function is anappropriate measurement for the similarity of the present breathingcycle with preceding breathing cycles. The top graph shown in FIG. 1thereby illustrates the breathing gas flow of a patient during NREM2sleep stage. The high vertical line of the middle graph indicates theend of inspiration, the lower vertical line of the middle graphindicates the end of expiration. The first derivation of the flow graphwhich allows detection of the end and the beginning of inspiration isillustrated as the lower graph. Because of the different extrema of thefirst derivation of the flow graph it is possible to reliablydistinguish between individual breathing phases.

The FIG. 2 graph illustrates as an example a 50-second portion ofrespiratory flow of a patient during NREM 2. The middle graph is aselected breathing cycle. The lower graph illustrates the correlationbetween the data sequence (top graph) and said selected breathing cycle.The correlation graph assumes values between 1 and −1, wherein thecorrelation assumes the value 1 in case that both breathing cyclescorrespond to each other exactly, —and the correlation assumes the value−1 when the graphs are correlated negative i.e. a top section ofbreathing pattern exactly meets a valley section of the analyzed dataportion.

On the basis of the correlation graph it is at first evident whetherrespiration is regular and at second whether breaths are missedcompletely. In case where successive breathing cycles are similar thegraph of correlation will have a periodic course with local maxima closeto 1 and local minima close to −1.

With respect to the correlation graph illustrated in FIG. 2 thedifference to the value 1 is calculated at each local maxima, whereinall of the thus obtained values are averaged. This average value between0 and 1 may be used as a measurement in how far the breathing patterncorresponds to the preceding breathing cycles.

In FIG. 3 the top graph illustrates the flow graph of a patient duringNREM 2 sleep stage. The lower data sequence illustrates the averagedifference of the maxima of correlation to the value 1.

FIG. 4 basically corresponds to FIG. 3 however the flow graph hereresults from REM-sleep stage. The comparison of the average maxima ofcorrelation according to FIGS. 3 and 4 shows that the average differenceof the maxima of correlation to 1 in REM sleep stage is clearly greater.

The following table illustrates which groups of respiratory states couldbe differenciated on the basis of the measurement of similarity as setforth above.

stable respiration non stable respiration silent regular respirationirregular respiration during REM respiration with associated snoringobstructive apnea mouth breathing awake respiration Periodic breathingwith flow limitation Cheyne Stoke respiration dampened respiratory flowsignalDetection of Snoring

The detection of snoring may be effected according to a preferredembodiment of the invention on the basis of the variance of theCPAP-pressure within a breathing cycle. In FIG. 5 the top graphillustrates a portion of a respiratory flow signal, there under there isillustrated the corresponding CPAP-pressure. The graph bottomillustrates the variance of the CPAP-pressure per breath. Said varianceclearly increases when the CPAP-Signal is varied due to patientssnoring.

Further Parameters of Discernment

The reliable detection of the transition points for beginning and end ofinspiration on the basis of the concept according to the presentinvention allows to retrieve further significant features fordistinguishing of breathing states. Particular advantageouslyretrievable indications are the time of inspiration, the time ofexpiration, the maximum flow during inspiration, the maximum flow duringexpiration, the volume of inspiration and the volume of expiration.

Mouth Breathing

artefacts due to mouth-breathing may be reliably detected since in thatcase a negative correlation is existing. Obstructive apneas might bedetected also in that certain peaks of correlation occur in a clearlyweakened manner—or are completely missing in the regular case.

Flow Limited Breathing

On the basis of the concept underlying the present invention a flowlimited breathing may be made out via the volume of inspiration or therelative change of the maximum inspiratory flow, in as far asinspiration is flow limited. If the approximate moments for beginningand end of inspiration are known it is possible to determine the momentof maximum inspiration. If this moment is placed in the first half ofinspiration the presence of a flow limited inspiration may be assumedwith high statistical likelihood and a respective correction of therespiratory pressure may be administered.

Cheyne Stoke Respiration

A periodic course of respiration showing periodic course of theinspiratory volume is characterizing Cheyne Stoke respiration which isthus distinguishable from other non stable breathing patterns.

Surveillance of the Detected Breaths

In a quite advantageously manner the correlation curve may be used forsurveillance of the detected moments of beginning and end ofinspiration, since a local maxima in the correlation graph isrepresenting with high statistical safety a feature of a breath.

On the basis of the concept of analysis underlying the device accordingto the present invention it is possible to detect individual breath withhigh statistical likelihood and to make far reaching conclusions withrespect to the present condition of the patient. Via the thus obtaineddetections it will become possible to adjust the therapy pressure in apredictive manner and with comparatively small changing-gradients inline with the physiological needs of the patient. This affords to a highacceptance of therapy.

What is claimed is:
 1. A device for detecting a person's breathing activity and/or physiological condition, comprising: a flow sensor configured to generate a first signal indicative of breathing gas flow over at least two breathing cycles; and at least one processor configured to: designate a limited portion of the first signal as a reference signal, compare the reference signal with at least a portion of the first signal, generate, based on the comparison, an output signal indicative of the breathing activity and/or the physiological condition of the person, and mathematically correlate the reference signal with the first signal as the comparison, wherein the mathematical correlation results in values between −1 and
 1. 2. The device of claim 1, wherein the at least one processor is further configured to determine whether successive breathing cycles are similar by determining whether a graph of the mathematical correlation in corresponding portions has a similar periodicity, and whether in the corresponding portions local maxima are close to 1 and local minima are close to −1.
 3. The device of claim 1, wherein the at least one processor is further configured to recognize whether the person's respiration is regular.
 4. The device of claim 1, wherein the at least one processor is further configured to recognize whether the person's breaths are missed completely.
 5. The device of claim 1, wherein the at least one processor is further configured to: generate a signal corresponding to the first derivative of the first signal, identify local maxima and minima of the signal corresponding to the first derivative of the first signal, recognize transitions between inspiration and expiration with reference to the local maxima and minima, and distinguish between individual breathing phases in accordance with the recognized transitions.
 6. The device of claim 1, wherein the at least one processor is further configured to cause a flow generator to adjust an amount of pressure provided to the person in dependence on the output signal.
 7. The device of claim 1, wherein the first signal extends over at least three breathing cycles, and wherein the reference signal corresponds to at least two breathing cycles.
 8. The device of claim 7, wherein the reference signal corresponds to exactly two breathing cycles.
 9. The device of claim 1, further comprising a filter configured to alter the first signal by removing or suppressing a predetermined frequency range therein.
 10. The device of claim 1, wherein the at least one processor is further configured to smooth the reference signal in accordance with smoothing criteria.
 11. The device of claim 10, wherein the smoothing criteria are adaptively changeable based on a detected breathing state.
 12. The device of claim 10, wherein the smoothing criteria are selected from preset smoothing criteria for respective respiratory states.
 13. The device of claim 1, wherein the at least one processor is further configured to compare the reference signal with at least a portion of the first signal received after the reference signal has been designated.
 14. The device of claim 1, wherein the value of 1 is reached when the reference signal and the first signal exactly correspond to one another, and wherein the value of −1 is reached when the reference signal and the first signal are exactly opposed to one another.
 15. A device for detecting a person's breathing activity and/or physiological condition, comprising: a flow sensor configured to generate a first signal indicative of breathing gas flow over at least two breathing cycles; and at least one processor configured to: designate a limited portion of the first signal as a reference signal, compare the reference signal with at least a portion of the first signal, and generate, based on the comparison, an output signal indicative of the breathing activity and/or the physiological condition of the person, wherein the portion of the first signal to which the reference signal is compared occurs prior to a time at which the reference signal is designated.
 16. A device for detecting a person's breathing activity and/or physiological condition, comprising: a flow sensor configured to generate a first signal indicative of breathing gas flow over at least two breathing cycles; and at least one processor configured to: monitor a second signal indicative of an amount of pressure provided to the person via a flow generator, designate a limited portion of the first signal as a reference signal, compare the reference signal with at least a portion of the first signal, generate, based on the comparison, an output signal indicative of the breathing activity and/or the physiological condition of the person, and mathematically correlate the reference signal with the first signal as the comparison, wherein the mathematical correlation results in values between −1 and
 1. 17. The device of claim 16, wherein the at least one processor is further configured to cause the flow generator to adjust the amount of pressure provided to the person in dependence on the output signal.
 18. The device of claim 16, further comprising a filter configured to alter the first signal by removing or suppressing a predetermined frequency range therein.
 19. The device of claim 16, further comprising a filter configured to alter the second signal by removing or suppressing a predetermined frequency range therein.
 20. The device of claim 16, further comprising a filter configured to alter the first and second signals by removing or suppressing a predetermined frequency range therein.
 21. The device of claim 16, wherein the at least one processor is further configured to identify snoring by detecting a increase in a variance of the second signal in one or more breathing cycles.
 22. The device of claim 16, wherein the at least one processor is further configured to identify mouth breathing by detecting a negative correlation.
 23. The device of claim 16, wherein the at least one processor is further configured to identify obstructive apneas by detecting a weakened and/or missing peak of correlation.
 24. The device of claim 16, wherein the at least one processor is further configured to identify flow limited breathing by determining whether a maximum inspiration flow occurs within the first half of an inspiration cycle.
 25. The device of claim 16, wherein the value of 1 is reached when the reference signal and the first signal exactly correspond to one another, and wherein the value of −1 is reached when the reference signal and the first signal are exactly opposed to one another.
 26. A method for detecting a person's breathing activity and/or physiological condition, comprising: generating a first signal indicative of breathing gas flow over at least two breathing cycles; designating a limited portion of the first signal as a reference signal; comparing the reference signal with at least a portion of the first signal; and generating, based on the comparison, an output signal indicative of the breathing activity and/or the physiological condition of the person, wherein the comparing comprises mathematically correlating the reference signal with the first signal, and wherein the mathematical correlation results in values between −1 and
 1. 27. The method of claim 26, further comprising determining whether a graph of the mathematical correlation in corresponding portions has a similar periodicity, and whether in the corresponding portions local maxima are close to 1 and local minima are close to −1, in order to determine whether successive breathing cycles are similar.
 28. The method of claim 26, further comprising recognizing whether the person's respiration is regular.
 29. The method of claim 26, further comprising recognizing whether the person's breaths are missed completely.
 30. The method of claim 26, further comprising: generating a signal corresponding to the first derivative of the first signal; identifying local maxima and minima of the signal corresponding to the first derivative of the first signal; recognizing transitions between inspiration and expiration with reference to the local maxima and minima; and distinguishing between individual breathing phases in accordance with the recognized transitions.
 31. The method of claim 26, further comprising causing a flow generator to adjust an amount of pressure provided to the person in dependence on the output signal.
 32. The method of claim 26, wherein the first signal extends over at least three breathing cycles, and wherein the reference signal corresponds to at least two breathing cycles.
 33. The method of claim 32, wherein the reference signal corresponds to exactly two breathing cycles.
 34. The method of claim 26, further comprising removing or suppressing a predetermined frequency range in the first signal.
 35. The method of claim 34, further comprising adaptively changing the smoothing criteria based on a detected breathing state.
 36. The method of claim 26, further comprising smoothing the reference signal in accordance with smoothing criteria.
 37. The method of claim 32, wherein the smoothing criteria are selected from preset smoothing criteria for respective respiratory states.
 38. The method of claim 26, wherein the portion of the first signal to which the reference signal is compared occurs prior to a time at which the reference signal is designated.
 39. The method of claim 26, wherein the comparing comprises comparing the reference signal with at least a portion of the first signal received after the reference signal has been designated.
 40. The method of claim 26, wherein the value of 1 is reached when the reference signal and the first signal exactly correspond to one another, and wherein the value of −1 is reached when the reference signal and the first signal are exactly opposed to one another.
 41. A method for detecting a person's breathing activity and/or physiological condition, the method comprising: generating a first signal indicative of breathing gas flow over at least two breathing cycles; monitoring a second signal indicative of an amount of pressure provided to the person via a flow generator; designating a limited portion of the first signal as a reference signal; comparing the reference signal with at least a portion of the first signal; and generating, based on the comparison, an output signal indicative of the breathing activity and/or the physiological condition of the person, wherein the mathematically comparing comprises mathematically correlating the reference signal with the first signal, and wherein the mathematical correlation results in values between −1 and
 1. 42. The method of claim 41, further comprising causing the flow generator to adjust the amount of pressure provided to the person in dependence on the output signal.
 43. The method of claim 41, further comprising filtering the first signal by removing or suppressing a predetermined frequency range therein.
 44. The method of claim 41, further comprising filtering the second signal by removing or suppressing a predetermined frequency range therein.
 45. The method of claim 41, further comprising filtering the first and second signals by removing or suppressing a predetermined frequency range therein.
 46. The method of claim 41, further comprising identifying snoring by detecting a increase in a variance of the second signal in one or more breathing cycles.
 47. The method of claim 41, further comprising identifying mouth breathing by detecting a negative correlation.
 48. The method of claim 41, further comprising identifying obstructive apneas by detecting a weakened and/or missing peak of correlation.
 49. The method of claim 41, further comprising identifying flow limited breathing by determining whether a maximum inspiration flow occurs within the first half of an inspiration cycle.
 50. The method of claim 41, wherein the value of 1 is reached when the reference signal and the first signal exactly correspond to one another, and wherein the value of −1 is reached when the reference signal and the first signal are exactly opposed to one another. 